nongreedy:
We specify the keyword in the user_agent string from right to left, and this is more efficient. As usual, we use the greedy algorithm. It will return immediately after the keyword being found.

E.g 1. "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)", this string is MSIE's user_agent string, we will return when we find the keyword "MSIE". But the truth is not alway like this:
E.g 2. "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_3) AppleWebKit/535.20 (KHTML, like Gecko) Chrome/19.0.1036.7 Safari/535.20", This is Chrome's user_agent. We will match Safari frist. If we define safari is greedy, it scans the string in a reverse order. If a keyword is greedy, it will not return when it matches the keyword at the first time. It will continue to scan the string.

default:
set the default value of this variable;

The directive format is like this in the block:
name version value;

name: the name of operating_system, browser, crawler and so on;
version: It can be omitted, and it support multiple formats;
value: It is the value filled to the variable;